The present disclosure relates to image reading devices, image forming apparatuses, and image reading methods.
An image reading device is used as a scanner and also as an image forming apparatus, such as a multi-function peripheral, a copier, and a facsimile machine. An image reading device reads an image of an original document by a photodetector detecting light reflected from the surface of the original document that is illuminated with light emitted by a light source.
Even when the image reading device reads the same original document, the amount of light detected by the photodetector may differ for each reading operation. For example, the amount of light detected by the photodetector may fluctuate depending on changes in the ambient conditions or in the light source, the optical system and/or the photodetector. In view of this, an image reading device typically applies shading correction which uses the result obtained by reading a white reference plate with a predetermined amount of light. Such shading correction reduces the fluctuations in the reading results caused by the changes in the amount of light.
Some image reading devices can read color images in addition to monochrome images. In the case where an image reading device is provided with a red light source, a green light source, and a blue light source, the photodetector can read color images in addition to monochrome images, by detecting red, green, and blue light respectively emitted by the red, green, and blue light sources.
For example, when the photodetector is a monochrome sensor, the red, green, and blue light sources are lighted at different times from one another in the color mode. In the monochrome mode, on the other hand, the red, green, and blue light sources are lighted at the same time to shorten the reading time. In this case, for increasing the dynamic range of the photodetector, the amounts of red, green, and blue light emitted by the red, green, and blue light sources are set separately for the color mode, which is for reading color images, and the monochrome mode, which is for reading monochrome images.
Some image reading devices set the amounts of red, green, and blue light for reading monochrome images by using the amounts of red, green, and blue light set for reading color images. In monochrome reading, some image reading devices increase the amounts of red, green, and blue light by increasing the amount of electric current supplied to each of the red, green, and blue light sources by the same percentage in order to reduce the influence of the decrease in the amount of light emission over time.